As a background technique of the invention, there is a technique described in PTL 1. PTL 1 describes an accelerator, as an accelerator capable of efficiently emitting ion beams having different energies, including a return yoke and a vacuum container, in which an incidence electrode is disposed closer to an incidence side of a beam emitting path in the return yoke than a central axis of the vacuum container, magnetic poles are radially disposed from the incidence electrode around the incidence electrode in the return yoke, and concave parts are alternately disposed with the magnetic poles in a circumferential direction of the return yoke; and in the vacuum container, a track concentric region is formed in which a plurality of beam circulating tracks are centered on the incidence electrode, and around the track concentric region, a track eccentric region is formed in which a plurality of beam circulating tracks are decentered from the incidence electrode, the beam circulating tracks are dense between the incidence electrode and an incidence of the beam emitting path in the track eccentric region, and an interval between the beam circulating tracks is wider on a 180° opposite side of the incidence of the beam emitting path with the incidence electrode as a base point.